1. Field of the Invention
The present invention is utilized in an optical communication. Particularly, the present invention relates to a technique for suppressing degradation of signal quality due to input signal power variation.
2. Description of Related Art
In the WDM (Wavelength Division Multiplex) optical transmission system, a plurality of signal channels having different wavelengths are transmitted simultaneously through an optical fiber. Therefore, the WDM optical transmission system is advantageous in that transmission capacity per optical fiber can be increased by increasing the number of wavelengths, saving the number of optical fibers.
The WDM optical transmission system mainly consists of (1) an optical transmitter provided at the transmitting end of an optical transmission line, (2) an optical receiver provided at the receiving end thereof and (3) the optical transmission line between the optical transmitter and the optical receiver. The optical transmission line usually consists of (A) a plurality of optical amplifiers (for example, erbium-doped optical fiber amplifiers) for amplifying WDM light signal and (B) a plurality of optical fibers for connecting the optical amplifiers mutually.
Signal quality of an optical signal transmitted from the optical transmitter degrades while traversing through the optical transmission line. The degradation of signal quality is caused mainly by ASE (Amplified Spontaneous Emission) and nonlinear waveform distortion. ASE is a constant amount of random light produced by the optical amplifier and nonlinear waveform distortion is a waveform distortion caused by time-dependent nonlinear refractive index changes in the transmission fiber induced by signal light pulses. The distortion increases with the signal light power.
The signal degradation due to ASE tends to increase with decreasing signal power at the output of the optical transmission line. This is because the input of an optical amplifier is usually connected to the output of the optical transmission line. Since the optical amplifier usually generates a constant amount of optical noise, ratio of signal light to ASE, that is, signal-to-noise ratio (SNR) degrades when signal power at the output of the optical transmission line decreases. That is, in order to reduce influence of ASE on the signal light, larger signal power is preferable at the output of the optical transmission line.
The degradation of signal quality due to nonlinear waveform distortion tends to increase with increasing signal power at the input of the optical transmission line. This is because nonlinear refractive index variation, which is the cause of nonlinear waveform distortion, increases with increase in signal power. Therefore, in order to suppress the influence of nonlinear waveform distortion on the signal quality, smaller signal power is preferable at the input of the optical transmission line.
Therefore, for suppressing the signal quality degradations from both ASE and nonlinear waveform distortion, larger signal power is preferable at the output of the optical transmission line while smaller input power is preferable at the input of the transmission line. However, since the output signal light power and the input signal light power are in one-to-one relation, the difference of the two powers being the loss of the optical transmission line, the above two conditions cannot be satisfied simultaneously. Therefore, the powers of the output and input signal lights are set so as to minimize the total degradation.
Even with the optimal signal power setting, it is impossible to completely remove the two degradations. Further, since both of the degradations accumulates while the signal light passes through a plurality of optical amplifiers and optical transmission fibers, maximum transmission distance, that is, length of the optical transmission system is limited by the accumulated degradations.
In order to relax this limitation on the maximum transmission distance, a method and a system utilizing Raman amplification has been proposed. Raman amplification is a phenomenon in which a signal light is amplified within an transmission fiber when light, which is referred to as pump light having specific wavelength different from that of the signal light, are launched simultaneously into the optical fiber.
There are three configurations for a system using Raman amplification: “backward pumping” configuration in which amplification is performed by using a pump light (referred to as backward pump light) propagating in the opposite direction to the signal light; the “forward pumping” configuration in which amplification is performed by using a pump light (referred to as forward pump light) propagating in the same direction as the signal light; and “bidirectional pumping” configuration in which amplification is performed by using both the backward pump light and the forward pump light. In any of these three system configurations, it is possible to improve the signal quality compared to systems which does not utilize Raman amplification, by appropriately setting the optical signal power at the input terminal of the optical transmission line.
For example, an improvement in signal quality by utilizing the backward pumping will be described with reference to FIG. 27. Curve (a) in FIG. 27 shows the signal power distribution in an optical transmission fiber in an optical communication system without Raman amplification. The average power of all wavelength channels is employed for simplicity. When the backward pump light is injected into the optical transmission line from the output terminal, the signal light is amplified in the optical fiber, as shown by curve (b) in FIG. 27. As a result, signal light power at the output of the optical transmission fiber is increased, reducing the influence of ASE generated by an optical amplifier connected to the output terminal. Therefore, the signal quality is improved.
An improvement in signal quality by utilizing the forward pumping will be described with reference to FIG. 28. Curve (a) in FIG. 28 shows the signal power distribution in an optical transmission fiber in an optical communication system without Raman amplification. When the forward pump light is injected into the optical transmission line from the input terminal, the signal light is amplified as shown by curve (b) in FIG. 28. When the input signal light power is reduced such that the signal light at the output of the optical transmission fiber becomes equal to the level before Raman amplification is applied, the output signal light becomes as shown by curve (c) in FIG. 28. As a result, it becomes possible to lower signal light power while maintaining the output signal power from the optical transmission line, that is, while maintaining the degradation due to ASE constant. Therefore, the nonlinear waveform distortion can be reduced leading to improved signal quality. When the bidirectional pumping configuration is used, it becomes possible to reduce the influences of both the ASE and nonlinear waveform distortion, simultaneously.
Such optical transmission system in which the signal quality is improved by Raman amplification is disclosed in, for example, H. Suzuki et al, Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication '99 Technical Digest, ThO4.
It has been known, however, that the Raman amplification improves the signal quality by the above mentioned mechanism on one hand but generates noise peculiar to Raman amplification on the other. Noise peculiar to Raman amplification is, for example, Raman ASE. Since Raman ASE rapidly increases with increase in signal gain by Raman amplification, maximum gain of Raman amplification, which effectively works in improving the signal quality, is limited by Raman ASE.
Assuming the input signal power at the input of an optical transmission fiber is the same and the gain of Raman amplification is the same, an amount of Raman ASE generation is minimum in the forward pumping configuration and maximum in the backward pumping configuration. The amount of Raman ASE generated in the bidirectional pumping configuration is in between the other two configurations.
Degree of signal quality degradation caused by Raman ASE increases with the decrease in the input signal light power into an optical transmission fiber. Therefore, if Raman amplification is used with low input signal power into an optical transmission fiber, degradation due to Raman ASE becomes substantial.
Noise peculiar to Raman such as Raman ASE amplification is disclosed in, for example, P. B. Hansen et al, IEEE Photonics Technology Letters, Vol. 10, No. 1, January 1998, pp.159–161.
Further, since the average signal power inside the optical transmission fiber is increased by Raman amplification, additional nonlinear waveform distortion occurs compared to the system without Raman. If the input signal power at the input of an optical transmission fiber is the same and the gain of Raman amplification is the same, the additional nonlinear waveform distortion is maximum in the forward pumping configuration and Raman amplification is minimum in the backward pumping configuration. The additional nonlinear waveform distortion in the bidirectional pumping configuration is in between the other two constructions.
If gain is obtained by forward pumping without lowering input signal light into the optical transmission line as shown by curve (b) in FIG. 28, the signal light power is higher at any longitudinal position of the optical transmission line, compared to the case without Raman amplification. Therefore, the nonlinear waveform distortion becomes larger compared to the case without Raman amplification.
Incidentally, when the light power is increased only in a later part of the optical transmission fiber in which signal light power is already lowered by loss, and when increase is small as in the case shown by curve (b) in FIG. 27, the additional nonlinear waveform distortion is negligibly small.
Although Raman amplification is accompanied with noise and nonlinear waveform distortion peculiar thereto, it is possible to improve the signal quality compared with the case without Raman amplification by properly designing to suppress influence of these matters.
Further, it is known that, when Raman amplification is utilized, it is possible to compensate, simultaneously with the improvement in signal quality, for the tilting of signal light power with respect to wavelength (wavelength dependency) that originates from propagating through an optical transmission fiber having wavelength-dependent loss. Since the gain characteristics of Raman amplification has wavelength dependency, it is possible to realize gain characteristics opposite to the loss wavelength characteristics of the optical transmission fiber by appropriately selecting wavelength and power of pump light.
Therefore, by using an optical transmission fiber as Raman amplification medium, it becomes possible to maintain at the output of the transmission fiber constant with respect to wavelength. By maintaining WDM signal power constant with respect to wavelength, it is possible to provide uniform signal quality in all wavelength channels.
Particularly, it is possible to compensate for the tilting of signal light power with respect to wavelength, which varies with time, by changing the pump light wavelength and/or the pump light power with time. An example of an optical communication system in which the loss wavelength dependency is compensated for by Raman amplification is disclosed in JP2001-7768A.
In JP2001-7768A, it is described that, when Raman amplification is used, it is preferable to control gain of Raman amplification such that signal light power at the output terminal of an optical fiber transmission fiber becomes uniform with respect to wavelength while at the same time the total output signal light power (total of signal light powers in all wavelength channels thereof) is maintained. This is because when an optical amplifier such as an erbium-doped fiber amplifier is connected to the output of the transmission fiber, change in total signal power would lead to gain tilt provided by the amplifier. When the gain is tilted with respect to wavelength, it becomes impossible to provide uniform signal quality over all wavelength channels.
As mentioned above, it is possible to improve the signal quality in an optical transmission system and to compensate for the loss wavelength dependency of the transmission fiber, which varies with time, by utilizing the conventional technique related to Raman amplification. However, the conventional technique has a problem when input signal power into the optical fiber transmission fiber is changed while the wavelength dependency is kept constant.
Input signal power into may change while wavelength characteristics is kept constant, for example, when the output signal power of an optical amplifier reduces due to trouble with a pumping laser diode, or when the loss of an transmission fiber changes due to reconnection of optical fiber transmission fibers by an optical cross-connect. The optical cross-connect may be of an automatic type using optical switches or of a manual type in which an optical patch panel is manually switched. Further, there may be a case when the system is reconnected to a different set of transmission fibers with different loss after turning off the system for the purpose of system recycling.
For example, in an optical communication system with backwardly pumped Raman amplification which initially operates in the optimal signal light power with which signal degradation is minimum as shown in curve (a) in FIG. 29, when the signal light power is lowered over all wavelength channels as shown by curve (b) in FIG. 29 (the average signal light power of the all wavelength channels is shown in FIG. 29), a control is conventionally performed such that the output power from the optical transmission fiber becomes constant. That is, the control increases pumping light power, so that gain due to Raman amplification is increased. As a result, the average signal power becomes as shown in curve (c) in FIG. 29.
In such state, since the output signal power from the transmission fiber is kept constant by controlling Raman gain, there is no variation in degradation due to ASE generated by a subsequent optical amplifier. However, as mentioned previously, degradation due to Raman ASE increases with decrease in input signal light into the transmission fiber. Therefore, if reduction of signal light power over the whole wavelength channels is large, degradation due to Raman ASE becomes substantial.
This will be described by using the simulation result shown in FIG. 31 and FIG. 32a to FIG. 32c. The abscissa in FIG. 31 is the distance, that is, position along transmission fiber and the ordinate is the average signal light power. Assuming that, initially, an optical communication system operates with the signal light power shown by curve (a) in FIG. 31 and the transmission fiber is a 125 km long single mode fiber, calculated waveform at the input and the output of the transmission fiber for a representative wavelength channel is as shown by waveforms in FIG. 32a and FIG. 32b, respectively. Signal bitrate is 40 Gbit/sec. From these figures, signal degradation due to ASE and nonlinear waveform distortion may be observed. When the signal light power is lowered over all wavelength channels as shown by curve (b) in FIG. 31, a conventional control tried to keep the output power from the transmission fiber constant. That is, in the conventional technique, the control increased the pump power, so that gain due to Raman amplification is increased. As a result, the average signal power distribution becomes as shown by curve (c) in FIG. 31. In this example, the input signal light power is lowered by 10 dB. As a result of the reduction of the input signal light power, the degradation of signal quality due to Raman ASE is increased and the waveform of the signal output from the transmission fiber becomes as shown by the waveform in FIG. 32c. From this, it is clear that degradation due to Raman ASE increased. Further, similar degradation occurs even when the loss of the transmission fiber is increased by 10 dB by such as by switching of connection of the optical fiber.
In a case where the signal light power over all wavelength channels is increased as shown by curve (b) in FIG. 30 in an optical communication system utilizing forwardly pumped Raman amplification which initially operates with the optimal signal light power with which the degradation of signal quality is minimum as shown by curve (a) in FIG. 30, the conventional control is performed to keep the output signal power light from the transmission fiber constant. That is, the control decreases pumping power so that gain due to Raman amplification is decreased. As a result, the average signal light power becomes as shown by curve (c) in FIG. 30.
In this state, since power of the signal light ejected from the transmission fiber is kept constant by controlling Raman gain, the amount of degradation due to ASE generated by a subsequent optical amplifier is not changed. However, since light power is increased in the preceding part of the transmission fiber compared to the power distribution before the increase, additional nonlinear waveform distortion in this portion of the transmission fiber is substantial.
This will be described by using the simulation result shown in FIG. 33, FIG. 34a and FIG. 34b. The abscissa in FIG. 33 is the distance, that is, position along transmission fiber and the ordinate is the average signal light power. Assuming that, initially, the optical communication system operates with the signal light power shown by curve (a) in FIG. 33 and the transmission fiber is a 125 km long single mode optical fiber, the calculated waveform at the input and the output of transmission fiber for a representative wavelength channel become as shown by the waveform in FIG. 34a. Signal bitrate is 40 Gbit/sec. When the signal light power is increased over all wavelength channels as shown by curve (b) in FIG. 33, the conventional tried to keep the output signal light from the transmission fiber constant. That is, in the conventional technique, the control decreased the pumping power, so that gain due to Raman amplification is reduced. As a result, the average signal power becomes as shown by curve (c) in FIG. 33. In this example, input signal light is increased by 10 dB. As a result of the increase in the input signal light power, the signal degradation due to nonlinear waveform distortion is increased and the waveform of the output signal light from the transmission fiber becomes as shown by the waveform in FIG. 34b. From this, it is clear that the nonlinear waveform degradation has become large.
In order to suppress such nonlinear waveform distortion, it is necessary to restrict Raman gain such as shown by curve (d) in FIG. 30. In such case, the increase in nonlinear waveform distortion in the preceding part of the transmission fiber cancels out with the decrease in nonlinear waveform distortion in the subsequent part of the transmission fiber and becomes comparable to the amount of nonlinear waveform degradation as in (a) of FIG. 30. In this case, however, since the output signal power from the transmission fiber is lowered, the signal quality is degraded by ASE generated by the subsequent optical amplifier.
In the conventional techniques related to the bidirectional pumping, a control method has not clearly been defined for the case when the input signal light power into an transmission fiber varied while wavelength characteristics thereof is kept constant. If a simple control is performed for the reduction in input signal light power such that gain obtained by the forward pumping and gain obtained by the backward pumping are increased at the same rate, signal quality in the backward pumping portion is degraded due to Raman ASE. On the contrary, when the input signal power is increased, signal quality in the forward pumping portion is degraded due to the additional nonlinear waveform distortion.
JP2000-98433A discloses an output light power control means in which the input or output light is monitored and a pumping light power is controlled on the result of the monitoring such that the output light power is kept at a predetermined value. However, JP2000-98433A does not disclose any control method, which takes the ratio of the forward pumping to the backward pumping and Raman ASE into consideration.
As such, since there is no control technique related to collaboration of the forward pumping and the backward pumping in the conventional optical communication system or an optical amplifier, which utilizes Raman amplification and because there is no optical amplifier, which utilizes Raman amplification that is capable of adapting to various input signal light powers, it has been usual to find the optimal point in try-and-error manner every time when the conventional optical amplifier is installed. Therefore, a huge amount of labor is required in installing the conventional optical amplifier and a cost for sending engineers to the location of the installation, causing such technique to be not realistic.
Further, if the characteristics of transmission line is changed because the configuration of the transmission line is changed such as by an optical cross-connect, it is difficult to flexibly adapt to such situation by the conventional technique.